1. Field of Invention
This invention relates to a thin film formation method by an ink jet method, an ink jet apparatus, a production method of an organic EL (electro-luminescence) device, and an organic EL device.
2. Description of Related Art
An organic EL display including organic EL devices (light emitting device having a light emitting layer made of an organic material interposed between an anode and a cathode) so arranged as to correspond to pixels has been rapidly developed in recent years as a spontaneous light emitting display that will replace current liquid crystal displays. Materials of the light emitting layer of the organic EL device can include aluminum quinolynol complexes (Alq3) and poly(paraphenylene)vinylene (PPV) as an organic material having a high molecular weight.
As disclosed in “Appl. Phys. Lett.” 51(12), 21 Sep., 1987, 913, it is known to form a film of a light emitting layer made of an organic material having a low molecular weight by vacuum evaporation. Another reference, “Appl. Phys. Lett.” 71(1), 7 Jul., 1997, p.34 et seq. describes the film formation of a light emitting layer made of an organic material having a high molecular weight.
In organic EL devices for display, it is necessary to form an anode at each pixel position on a substrate and to dispose a light emitting layer on each anode. If the arrangement of the light emitting layer can be performed by an ink jet method, precise patterning can be made within a short time because application and patterning can be conducted simultaneously. Moreover, since the amount of materials to be used is that which is minimum necessary, the materials can be used without waste and the production cost can be lowered.
To arrange the light emitting layer by an ink jet method, it is necessary to use a material in the liquid form. When a polymer material such as PPV is used as the material of the light emitting layer, the arrangement can be made by the ink jet method if a precursor solution of the polymer material is used. Japanese Patent Laid-Open Publication Nos. 11-40358, 11-54270 and 11-339957 teach to arrange a light emitting layer made of a PPV type polymer material in accordance with the ink jet method.
As shown in FIG. 1A, in the liquid arrangement by the conventional ink jet method, an ink jet head 2 smaller than a substrate 1, for example, is employed. The inside of the surface of the substrate 1 is so divided into a plurality of regions 11 to 15 as to correspond to the length of rows of nozzles 3 of the head 2. The liquid is serially discharged from the nozzles 3 of the head 2 while the substrate 1 or the head 2 is being moved.
According to this method, however, when a solvent of the liquid to be discharged is a solvent having a large density, a solvent vapor evaporating from droplets is likely to stay inside the substrate surface. When a droplet A having an early arrangement order on the substrate is compared with a droplet B having a late arrangement order, for example, as shown in FIG. 1B, the droplet B having a late arrangement order is discharged in an atmosphere in which the partial pressure of the solvent vapor is high. As a result, a drying rate of the droplet B is lower than that of the droplet A. The droplet A arranged previously, too, is affected by the solvent vapor staying inside the substrate surface and in some cases, it is again dissolved after drying or its drying rate becomes lower.
Therefore, when a solution prepared by dissolving a plurality of polymer materials having mutually different molecular weight or polarity in a solvent having a large density is arranged on the substrate by the conventional ink jet method, the droplets having a low drying rate are likely to result in a thin film in which a plurality of polymer materials are in the phase separation state. When the drying rates of the droplets are different inside the substrate surface, the condition of the resultant thin film becomes different depending on the position inside the substrate surface.
As described above, when the conventional ink jet method is employed to arrange the light emitting layer in the organic EL display, luminance is likely to vary inside and among pixels.